Reducing the Water and Water Vapour Absorbence and Enhancing the Dimensional Stability of Paper and Paper Products and Use of Coated Paper Products

ABSTRACT

The invention relates to a method for reducing the absorption of water and water vapor and for increasing the dimensional stability of paper and paper products by treatment with an aqueous solution and/or dispersion of at least one reactive material which reacts with itself and/or cellulose fibers with crosslinking, cellulose fibers or a paper product obtained therefrom by drainage on a wire being compressed, the compressed paper product then being brought into contact with an aqueous solution and/or dispersion of the reactive material, the compression being eliminated with further action of the aqueous solution and/or dispersion and the paper product being dried and crosslinked, and to the use of the coated paper products thus obtainable and/or of the coated cellulose fibers which can be produced therefrom by defibrating as an additive to thermoplastics and as an additive to heat-curable plastics.

The invention relates to methods for reducing the absorption of water and water vapor and for increasing the dimensional stability of paper and paper products by treatment with an aqueous solution and/or dispersion of at least one reactive material which reacts with itself and/or with the cellulose fibers with crosslinking, and heating of the treated materials to a temperature at which drying and crosslinking takes place, and the use of coated paper products and/or of the coated cellulose fibers which can be produced therefrom by defibrating as an additive to thermoplastics and as an additive to heat-curable plastics.

From the publication “Treatment of timber with water soluble dimethylol resins to improve the dimensional stability and durability”, which appeared in Wood Science and Technology 1993, pages 347-355, it is known that the shrinkage and swelling properties of wood and the resistance to fungi and insects can be improved by treating wood with an impregnating agent which consists of an aqueous solution of dimethylol dihydroxyethyleneurea (DMDHEU or 1,3-bis(hydroxymethyl)-4,5-dihydroxyimidazolidin-2-one) and a catalyst. Catalysts used are metal salts, citric acid and amine salts, individually or in combination. The DMDHEU is used in the aqueous solution in concentrations of from 5% to 20%. The added amount of catalyst is 20% based on the DMDHEU. The impregnation is effected under reduced pressure. At elevated temperature, a reaction of the DMDHEU with itself and with the wood takes place. This reaction takes place for one hour in a drying oven at temperatures of 80° C. or 100° C. The wood samples thus treated exhibit an improvement in the shrinkage and swelling properties of up to 75%, and do so at DMDHEU concentrations of 20%. In this way, wood bodies having dimensions of 20 mm×20 mm×10 mm were investigated. The method described can be used only in the case of small dimensions of the wood bodies, because these tend to crack in the case of larger dimensions.

EP-B 0 891 244 discloses the impregnation of wood bodies comprising solid wood with a biodegradable polymer, a natural resin and/or a fatty acid ester—if appropriate with application of reduced pressure and/or pressure. The impregnation takes place at elevated temperatures. The pores in the wood are at least largely filled, and a molding which comprises both wood and biodegradable polymer forms. The polymer does not react with the wood. With this treatment, the characteristic properties of wood, the biodegradability and the mechanical properties are not lost. The thermoplasticity can be increased. Depending on the proportion of polymer introduced, there is an increase in the surface hardness by the incorporation of the polymer into the wood matrix, so that naturally soft woods are also suitable for high-quality floors.

SE-C 500 039 describes a method for hardening wood with densification, in which untreated wood is impregnated with various aminoplast monomers based on melamine and formaldehyde by means of suitable vacuum pressure impregnation, then dried, and cured in a press for densification at elevated temperature. Agents mentioned are, inter alia, DMDHEU, dimethylolurea, dimethoxymethylurea, dimethylolethyleneurea, dimethylolpropyleneurea and dimethoxymethyluron. This method has the disadvantage that the natural wood structure is lost as a result of the densification, and the formaldehyde emission of the finished wood body is relatively high, depending on the crosslinking agent used.

WO 04/033171 discloses a method for the production of a wood body having high surface hardness and low formaldehyde emission, an untreated wood body being impregnated with an aqueous solution of

-   A) an impregnating agent consisting of a     1,3-bis(hydroxymethyl)-4,5-dihydroxyimidazolidin-2-one modified with     a C₁₋₅-alcohol, a polyol or mixtures thereof, and -   B) a catalyst from the group consisting of the ammonium or metal     salts, organic or inorganic acids and mixtures thereof,     dried, and then cured at elevated temperature.

According to the method disclosed in WO 04/033170 the durability, dimensional stability and surface hardness of a wood body is improved by impregnating a wood body with a 1 to 50% strength by weight aqueous solution of an impregnating agent comprising a substance of group A and/or at least one substance of group B and at least one substance of group C as a catalyst, and causing the impregnating agent subsequently to react with itself and with the wood under humid conditions for avoiding drying. Suitable impregnating agents are, for example, dimethyloldihydroxyethyleneurea (DMDHEU), urea-glyoxal adducts and urea-formaldehyde adducts. Suitable catalysts are, for example, magnesium chloride, zinc chloride, ammonium chloride or acids, such as formic acid, maleic acid, hydrochloric acid or sulfuric acid.

WO 2004/025019 discloses a method and an apparatus for exchanging a liquid present in fibers with another liquid. The procedure adopted here is to press out fiber cake to such an extent that a considerable amount of the liquid which is present in the fibers is transferred into the space between the fibers, to meter the other liquid, which is to replace the first liquid, into the compressed fiber cake during the compression step so that the first liquid is removed from the space between the fibers, and then to let down the pressure on the fiber cake under further action of the other liquid which is to replace the first liquid, further replacement liquid being absorbed. Liquid cleaners, chemical treatment agents, liquid acids or bases, bleaches, delignification agents, catalysts, complexing agents, fluorescence indicators, metal ions, cationic or anionic polymers, colorants and inorganic substances being mentioned as replacement liquid. With the aid of the known method, it is possible, for example, at least partly to remove lignin constituents from cellulose fibers and thus to prepare a pulp having a better and more uniform quality.

Cellulose fibers and paper products produced therefrom, such as paper, board and cardboard, readily absorb water and also water vapor from the air. As a result, however, the dimensional stability and the mechanical stability of the cellulose fibers and of the paper products are reduced to an undesired extent. In order to reduce the water absorption of paper products, a wet strength agent can be added, to the paper stock, for example during production of said products. Known wet strength agents are, for example, urea-formaldehyde resins, which increase not only the wet strength but also the dry strength of the paper (cf. EP-A 0 123 196 and U.S. Pat. No. 3,275,605), melamineformaldehyde resins (DE-B 10 90 078) and other commercially available products, for example epichlorohydrin-crosslinked condensates of polyamidoamines, such as the Luresin® brands (BASF Aktiengesellschaft).

It is the object of the invention to provide a method for reducing the absorption of water and water vapor and for increasing the dimensional stability of paper and paper products, such as board and cardboard.

The object of the invention is achieved, according to the invention, by a method for reducing the absorption of water and water vapor and for increasing the dimensional stability of paper and paper products by treatment with an aqueous solution and/or dispersion of at least one reactive material which reacts with itself and/or cellulose fibers with crosslinking, and heating of the treated materials to a temperature at which drying and crosslinking takes place, if cellulose fibers or a paper product obtained therefrom by drainage on a wire are or is first compressed, the compressed paper product is then brought into contact with an aqueous solution and/or dispersion of the reactive material, the compression is eliminated with further action of the aqueous solution and/or dispersion and the paper product is dried and crosslinked. The crosslinking of the reactive materials takes place, for example, at temperatures above 30° C., for example in the temperature range of from 35 to 200° C. The method can be carried out continuously and also batchwise.

A preferred procedure is one in which cellulose fibers which comprise at least 50% by weight of virgin fibers or a paper product obtained therefrom by drainage on a wire, having a water content of in each case at least 0.7 g of water by g of dry cellulose fibers, are or is first compressed under a pressure of at least 2.1 MPa, the compressed paper product is then brought into contact with an aqueous solution and/or dispersion of the reactive material, the compression is eliminated with further action of the aqueous solution and/or dispersion, and the paper product is dried and is heated to a temperature in the range of from 70 to 200° C. for crosslinking.

For example, the aqueous solution and/or dispersion comprises, as reactive material, at least one heat-curable binder from the group consisting of the urea-formaldehyde adducts, urea-glyoxal adducts, melamine-formaldehyde adducts, phenol-formaldehyde adducts, one- and two-component systems based on epoxy resins, polyurethanes or isocyanates, polyacrylates, polymethacrylates, styrene-(meth)acrylate copolymer dispersions and/or styrene-butadiene-(meth)acrylic acid copolymer dispersions. In some cases, the use of mixtures of at least two reactive materials is of interest, for example mixtures of melamine/urea-formaldehyde condensates. The reactive materials may be present as aqueous solution or as aqueous dispersion. Here, transitions between solution and dispersion are possible. If dispersions are used for example, the mean particle diameter of the polymer particles dispersed in water is less than 1 μm, preferably less than 500 nm and generally in the range of from 10 to 100 nm.

The aqueous solution and/or dispersion thus comprises, for example, a group of a reactive, crosslinkable material which may consist of

-   (i) at least one reactive substance which forms a polymer, -   (ii) if appropriate, at least one C₁₋₅-alcohol, at least one polyol     or mixtures thereof and -   (iii) at least one catalyst.

Examples of (i) a reactive substance which forms a polymer are urea-glyoxal adducts and derivatives thereof, e.g. 1,3-bis(hydroxymethyl)-4,5-dihydroxyimidazolidin-2-one (referred to below as “DMDHEU”). In the impregnation, it can be used either alone or together with (ii) at least one C₁₋₅-alcohol, a polyol or mixtures thereof. If 1,3-bis(hydroxymethyl)-4,5-dihydroxyimidazolidin-2-one is used together with an alcohol and/or polyol as the impregnating agent, correspondingly modified 1,3-bis(hydroxymethyl)-4,5-dihydroxyimidazolidin-2-ones (referred to below as “mDMDHEU”) form. Such compounds are disclosed, for example, in U.S. Pat. No. 4,396,391 and WO 98/29393. These are reaction products of 1,3-bis(hydroxymethyl)-4,5-dihydroxyimidazolidin-2-one with at least one C₁₋₅-alcohol, at least one polyol or mixtures thereof.

The compounds of group (ii) include C₁₋₅-alcohols, for example methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, tert-butanol and n-pentanol, preferably methanol, and polyols, such as ethylene glycol, diethylene glycol, 1,2- and 1,3-propylene glycol, 1,2-, 1,3- and 1,4-butylene glycol, glycerol, trimethylolpropane and polyalkylene glycols, such as polyethylene glycol, polypropylene glycol, block copolymers of ethylene glycol and propylene glycol. Polyethylene glycols of the formula HO(CH₂CH₂O)_(n)H, where n is from 3 to 20, and diethylene glycol are preferred.

In order to prepare modified 1,3-bis(hydroxymethyl)-4,5-dihydroxyimidazolidin-2-one (mDMDHEU), DMDHEU and the monohydric alcohol and/or the polyol are mixed, the monohydric alcohol and/or the polyol being used in an amount of from 0.1 to 2.0 mol equivalents each, based on DMDHEU. The mixture of DMDHEU, monohydric alcohol and/or polyol is reacted, for example, at temperatures of from 20 to 70° C. and a pH of from 1 to 2.5, the pH being adjusted to 4 to 8 after the reaction.

(i) a reactive substance which forms a polymer is to be understood as meaning both urea-formaldehyde adducts and urea-glyoxal adducts and in each case derivatives thereof. The following compounds may be mentioned by way of example: dimethyllolurea, bis(methoxymethyl)urea, tetramethylolacetylenediurea, methylolmethylurea and 1,3-dimethyl-4,5-dihydroxyimidazolidin-2-one, 1,3-bis(hydroxymethyl)imidazolidin-2-one or mixtures thereof. These compounds of group (i) can, if appropriate, also be used in the presence of (ii) at least one C₁₋₅-alcohol, at least one polyol or mixtures thereof as the impregnating agent. Suitable alcohols and polyols have already been mentioned above. Methanol, diethylene glycol and mixtures thereof are preferred.

The aqueous solution of the impregnating agent comprises the reactive compounds of group (i) and the compounds of group (ii), for example, in a concentration of from 1 to 70% by weight, preferably from 10 to 60% by weight and in particular from 20 to 60% by weight. The impregnating agent preferably comprises 1,3-bis(hydroxymethyl)-4,5-dihydroxyimidazolidin-2-one (DMDHEU) as a compound of group (i).

In addition to (i) and, if appropriate, (ii), the impregnating agent always comprises a catalyst (iii). Suitable catalysts (iii) are, for example, metal salts from the group consisting of metal halides, metal sulfates, metal nitrates, metal tetrafluoroborates, metal phosphates or mixtures thereof. individual examples of (iii) are magnesium chloride, magnesium sulfate, zinc chloride, lithium chloride, lithium bromide, boron trifluoride, aluminum chloride, aluminum sulfate, zinc nitrate and sodium tetrafluoroborate. Said compounds can be used, either alone or as a mixture, as a catalyst.

Further suitable catalysts (iii) are ammonium salts, such as ammonium chloride, ammonium sulfate, ammonium oxalate, diammonium phosphate or mixtures thereof. In addition, organic and/or inorganic acids may be used as a catalyst. Examples of these are maleic acid, formic acid, acetic acid, propionic acid, citric acid, tartaric acid, oxalic acid, p-toluenesulfonic acid, hydrochloric acid, sulfuric acid, boric acid or mixtures thereof.

Preferably used compounds of group (iii) are magnesium chloride, zinc chloride, magnesium sulfate, aluminum sulfate or mixtures of these compounds. Magnesium chloride is particularly preferred.

The catalyst (iii) is present, for example, in a concentration of from 0.1 to 10% by weight, preferably from 0.2 to 8% by weight, particularly preferably from 0.3 to 5% by weight, based on the components (i)-(iii) of the reactive material.

Of the products which are described above and comprise formaldehyde in the form of condensed units, in particular low-formaldehyde condensates are used. In the present context, low-formaldehyde is to be understood as meaning that the reactive materials comprise no substantial amounts of free formaldehyde and that no substantial amounts of formaldehyde are released even during drying or curing of the cellulose fibers or paper products treated therewith. In general, such reactive materials comprise <100 ppm of formaldehyde.

Further reactive materials which react with themselves and/or cellulose fibers with crosslinking are formaldehyde-free, heat-curable binders. Such binders are described, for example, in the following publications, which are hereby incorporated by reference as disclosure content of the present invention, namely U.S. Pat. No. 4,076,917, EP-A 0 445 578, EP-A 0 583 086, EP-A 0 651 088, WO 97/31036, page 4, line 12, to page 12, line 14, WO 97/31059, page 2, line 22, to page 12, line 5, WO-A-97/31060, page 3, line 8, to page 12, line 36, DE-A-199 49 591, page 3, line 5, to page 7, line 38, WO 01/27163, page 5, line 34, page 22, line 2, and the radiation-curable binders disclosed in DE-A 199 17 965.

In addition to the binders which are described in the abovementioned publications, suitable heat-curable binders are all curable binders which are described in the literature, for example, for strengthening nonwovens and/or are used for this purpose in practice, such as heat-curable resins based on phenol and formaldehyde, the abovementioned melamine-formaldehyde and urea-formaldehyde resins, urea-glyoxal resins and in particular formaldehyde-free one- and two-component systems based on epoxy resins or polyurethanes, polyacrylates, polymethacrylates, polyvinyl acetates, styrene acrylate copolymer dispersions, styrene-methacrylate copolymer dispersions, styrene-butadiene-(meth)acrylic acid copolymer dispersions and mixtures of said dispersions with a mixture of a polycarboxylic acid and a polyhydric alcohol as crosslinking component.

Examples of preferred heat-curable binders are mixtures of

-   (a) a polymer which is obtainable by free radical polymerization and     which comprises, incorporated in the form polymerized units, from 5     to 100% by weight of an ethylenically unsaturated carboxylic     anhydride or of an ethylenically unsaturated dicarboxylic acid whose     carboxyl groups can form an anhydride group, and -   (b) at least one alkanolamine, which comprises at least two hydroxyl     groups in the molecule and/or at least one polyhydric alcohol.

Specific examples of such mixtures are aqueous solutions and/or dispersions of a copolymer of 80% by weight of acrylic acid and 20% by weight of maleic acid, comprising from about 40 to 60% by weight of solids and having a molar mass M_(w) of from 15 000 to 900 000, in combination with triethanolamine or aqueous solutions of a copolymer of 55% by weight of acrylic acid and 45% by weight of maleic acid in combination with triethanolamine. These binders can comprise, if appropriate, an esterification catalyst and/or a compound comprising bound phosphorus, such as hypophosphorous acid, as a reaction accelerator.

The copolymer (a) described above may be composed, for example, of

-   -   from 50 to 99.5% by weight of at least one ethylenically         unsaturated mono- or dicarboxylic acid,     -   from 0.5 to 50% by weight of at least one ethylenically         unsaturated compound from the group consisting of the esters of         ethylenically unsaturated monocarboxylic acids and the         monoesters and diesters of ethylenically unsaturated         dicarboxylic acids with an amine having at least one hydroxyl         group and     -   up to 20% by weight of another monomer.

Heat-curable, aqueous compositions which comprise at least one copolymer (a) and at least one alkanolamine or higher-functional β-hydroxyalkylamine and/or at least one polyhydric alcohol can, if appropriate, additionally comprise at least one surfactant.

Further heat-curable binders are based on aqueous mixtures of

-   -   polycarboxylic acids, such as polyacrylic acid, polymethacrylic         acid, copolymers of acrylic acid and maleic acid, copolymers of         methacrylic acid and maleic acid, copolymers of ethylene and         maleic acid, styrene and maleic acid, or copolymers of acrylic         acid or methacrylic acid and esters of acrylic or methacrylic         acid with preferably monohydric alcohols comprising 1 to 24         carbon atoms, the polycarboxylic acids exhibit a K value of from         50 to 100 (measured with the polycarboxylic acids in         unneutralized form according to H. Fikentscher in         dimethylformamide at 25° C. and a polymer concentration of 0.1%         by weight) and     -   polyhydric alcohols, such as trimethylolpropane, glycerol,         2-hydroxymethylbutane-1,4-diol and polyvinyl alcohol, and/or         polyfunctional amines and/or alkanolamines.

Polycarboxylic acids, polyhydric alcohols, alkanolamines and polyfunctional amines are preferably used in amounts such that the number of acid functions is equivalent to the total number of alcoholic hydroxyl and amine functions, cf. EP-A 0 445 578. In addition, crosslinkable materials which consist of an aqueous solution of a polycarboxylic acid (homo- or copolymer), preferably having a molar mass M_(w) of 10 000 or less, and a polyol, such as triethanolamine, and in which the ratio of the number of equivalents of hydroxyl groups to the number of equivalents of carboxyl groups is in the range of from 0.4:1 to 1.0:1 are suitable, cf. EP-A 0 990 727.

In the method according to the invention, binders which are sold under the trade name Acrodur® by BASF Aktiengesellschaft are particularly advantageously used as reactive materials. An example of this is an aqueous styrene-acrylate polymer dispersion which is modified with a polycarboxylic acid and a polyhydric alcohol as crosslinking component. It crosslinks at a temperature of as low as 130° C. However, in order to achieve high production speeds, the crosslinking is preferably carried out at temperatures of from 180 to 200° C. A further formaldehyde-free binder is commercially available, for example, as a colorless to slightly yellowish, clear, aqueous solution of a modified polycarboxylic acid with a polyhydric alcohol as crosslinking component. It crosslinks, for example, at drying temperatures of from about 160 to 180° C.

Formaldehyde-free reactive materials which comprise at least one polycarboxylic acid and at least one polyhydric alcohol and/or alkanolamine or polyfunctional amine are particularly preferred. Compositions which comprise these reactive agents can, if appropriate, comprise even further formaldehyde-free polymers, e.g. polyacrylates, which are sold under the trade name Acronal® by BASF Aktiengesellschaft. The aqueous solutions and/or dispersions of a reactive material which are used for the impregnation comprise the reactive material, for example in an amount of from 1 to 70% by weight, preferably from 10 to 60% by weight and generally from 30 to 50% by weight.

In the context of the invention, paper products are to be understood as meaning, for example, paper itself and board and cardboard. For the method according to the invention, it is possible to start from cellulose fibers of all types, both from natural and from recovered fibers, in particular from fibers from waste paper, which, however, are used only as a mixture with virgin fibers. Virgin fibers are to be understood as meaning cellulose fibers which have not yet been processed to give a paper product or which have not yet been dried. In fiber mixtures comprising virgin fibers and fibers from waste paper, the amount of virgin fibers is, for example, at least 50% by weight, preferably at least 70% by weight. In the particularly preferred process variant, a pulp which comprises 100% of virgin fibers is used as a starting material Suitable fibers for the production of the pulps are all qualities customary for this purpose, e.g. mechanical pulp, bleached and unbleached chemical pulp and paper stocks from all annual plants. Mechanical pulp includes, for example, groundwood, thermomechanical pulp (TMP), chemothermomechanical pulp (CTMP), pressure groundwood, semi-chemical pulp, high-yield pulp and refiner mechanical pulp (RMP). For example, sulfate, sulfite and soda pulps are suitable as chemical pulp. Unbleached chemical pulp, which is also referred to as unbleached kraft pulp, is preferably used. Suitable annual plants for the production of paper stocks are, for example, rice, wheat, sugarcane and kenaf.

In contrast to dried cellulose fibers, virgin fibers have a high porosity, cf. W. Gindl, F. Zargar-Yaghubi and R. Wimmer, Bioresource Technology 87, 325-330 (2003). If a collection of moist cellulose fibers is considered, the water is found both between the individual cellulose fibers and in the interior of the cellulose fibers. An aqueous slurry of cellulose fibers is pressed during drainage on the wire of a paper machine to such an extent that the sheets formed therefrom comprise from 0.7 to 1.0 g of water per g of dry cellulose fibers, cf. G. V. Laivins and A. M. Scallan, TAPPI Proceedings, Engineering Conference, Book 2, 741-747 (1993). After pressing, for example with the aid of a size press, water is present in the spaces between the fibers and in the interior of the cellulose fibers. As long as the fibers have a sufficient porosity, water can also be removed from the interior of the fibers by pressing a fiber structure. In the method according to the invention, the paper product drained on a wire is subjected to a pressure such that water is forced out of the interior of the cellulose fibers. This pressure is at least 2.1 MPa and may be, for example up to 50 MPa. Preferably, it is in the range of from 2.5 to 10 MPa. As a result of the action of a pressure of at least 2.1 MPa on the moist fiber product comprising a predominant proportion of virgin fibers, the water content of the paper product is reduced to values below 0.7 g of water per g of dry fibers. It is, for example, from 0.3 to 0.5 g of water per g of dry cellulose fibers and is generally in the range of from 0.3 to 0.4 g of water per g of dry cellulose fibers.

The action of a pressure on the fiber structure and the treatment of the fiber structure with an aqueous solution of a reactive material can be effected continuously or batchwise. A continuous procedure is disclosed in WO 2004/025019, page 5, line 3, to page 8, line 8, mentioned in connection with the prior art. As explained in more detail there, a fiber cake on a wire or a belt is passed through a nip formed by two compression rolls and is compressed therein. As a result, a part of the water which is present in the cellulose fibers is forced out of the interior of the cellulose fibers into the spaces between the fibers of the compressed fiber cake and partly out of the fiber cake. With the aid of a compressible belt which revolves over a roll which, together with the other roll, forms the nip for the compression of the cellulose fiber cake, the compressed cellulose fiber structure is brought into contact under pressure with an aqueous solution of the reactive material. As a result, the water which originates from the interior of the cellulose fibers and is present in the intermediate spaces between the fibers is replaced by the aqueous solution of the reactive material. After leaving the roll nip, the compressed cellulose fiber structure is passed through an interstice which is filled with an aqueous solution of a reactive material. Relaxation of the compressed cellulose fibers begins. Similarly to a compressed sponge, which is released, cellulose fibers absorb aqueous solution of the reactive material. The solution penetrates not only into the intermediate spaces of the paper product but also into the interior of the cellulose fibers. In this way, not only coating of the individual cellulose fibers of the paper product with a reactive material but also at least partial coating of the interior of the fibers is achieved. After the treatment with an aqueous solution of a reactive material, the paper product is dried and is heated to a temperature of, for example, from 70 to 200° C. for crosslinking the reactive material.

In a batchwise embodiment of the method according to the invention, for example, it is possible to adopt a procedure in which first a paper machine wire having a mesh size of, for example from 80 to 150 μm and then a sheet which is produced from a predominant portion of virgin fibers and has a basis weight of, for example, from 50 to 500 g/m², in general from 75 to 250 g/m², and a water content of, for example, from 50 to 80% by weight are placed in a press equipped with a perforated tray. Thereafter, a papermaker's felt which is impregnated with an aqueous solution of at least one reactive material and then a sheet of plastic, e.g. polymethyl methacrylate, polystyrene or polypropylene, are placed in succession on the paper sheet. A pressure of at least 2.1 MPa is then exerted on the layers of said materials which are present in the press with the aid of a piston inserted into the press. Water which originates from the cellulose fiber structure and the interior of the cellulose fibers, together with excess aqueous solution of the reactive material is forced out of the perforated tray. The duration of the action of the pressure when the method according to the invention is carried out batchwise is, for example, from 0.1 to 120 seconds, preferably from 0.5 to 20 seconds. In the continuous procedure, the duration of the pressure is, for example, from 0.01 to 20 seconds, preferably from 0.02 to 1 second. After the end of the compression, the sheet absorbs further aqueous solution of the reactive material on relaxation. It is then removed from the press, and dried and heated to a temperature of, for example, from 70 to 200° C., preferably from 120 to 170° C., for crosslinking the reactive material.

While a polymer application of <5 g/m², in general from 1 to 3 g/m², is usually achieved in the case of application of an aqueous polymer solution with the aid of a size press, the polymer application in the method according to the invention is, for example, >5 g/m², e.g. from 5.5 to 8 g/m². In comparison with known application methods, paper and paper products which have a reduced absorption of water and water vapor and a higher dimensional stability are therefore obtained by the method according to the invention.

Suspensions of cellulose fibers can be produced from the paper products obtained by the method according to the invention, for example by disintegration of the paper or of the paper products in water, from which suspensions in turn it is possible to obtain, by removal of water, coated cellulose fibers which comprise the coating material at least partly in the interior. These cellulose fibers may be present, for example, in the form of a powder.

Both writing and printing papers and packaging papers, corrugated board, wallpapers, cardboard, laminates of, for example, a composite of board or paper and at least one film or sheet of a thermoplastic, and construction elements can be produced by the method according to the invention. Of particular interest are mixtures of (i) the paper products obtainable by the method according to the invention and/or the coated cellulose fibers which can be produced by defibrating and (ii) thermoplastics or heat-curable plastics. Moldings of any desired design can be produced from such mixtures.

The invention therefore furthermore relates to the use of the coated papers or paper products obtainable by the method according to the invention and/or the coated cellulose fibers which can be produced therefrom by defibrating as an additive to thermoplastics and as an additive to heat-curable plastics.

Such mixtures comprise, for example, from 0.1 to 90% by weight, preferably from 1 to 70% by weight and in general from 2 to 50% by weight of at least one component (i). The composite materials are prepared, for example, by mixing at least one of the coated materials with at least one thermoplastic or one heat-curable material. The mixing can be effected, for example, in an extruder, for example at least one product coated according to the invention and a thermoplastic being heated to a temperature which is in the respective softening range of the thermoplastic or higher and the mixture being extruded.

Suitable thermoplastics are, for example, polyolefins, such as polyethylenes, which are obtainable by the high-pressure or low-pressure polymerization process, polypropylene, polybut-2-ene or polybut-1-ene, polyisobutylene, polystyrene, polyamides, such as polycaprolactam or condensates of hexamethylenediamine and adipic acid, polyesters, such as polyethylene terephthalate, polymethyl methacrylate, polycarbonate and polyvinyl chloride.

Examples of heat-curable plastics are all reactive materials which have already been described above for the coating of paper and paper products, e.g. urea-formaldehyde resins, melamine-formaldehyde resins, one- and two-component systems based on epoxy resins, polyurethane or isocyanates, crosslinkable polyacrylates and crosslinkable polymethacrylates.

The mixtures of the components (i) and (ii) are suitable for the production of moldings, in particular for the production of construction elements, such as composites for the insulation of walls, as a water vapor barrier, in the form of sheets for the cladding of facades or in the interior for the production of doors and claddings, as material for the production of pieces of furniture which are used outdoors and inside, as housings for electrical appliances, such as vacuum cleaners, kitchen machines, televisions, radios, stereo units and computers, as material for automotive parts, for example interior door trims, dashboards and shelves for seats, as material for flower boxes, flowerpots, watering cans, plant tubs, walls and supporting parts for summer houses and for toys and as packaging material.

Unless otherwise evident from the context, the stated percentages in the examples are percentages by weight.

EXAMPLES Determination of the Water Absorption

Paper samples having the dimensions 4 cm×4 cm were weighed, then stored for 30 minutes in distilled water at a temperature of 20° C., then removed, dried with an absorbent cloth and weighed. The weight increase is calculated in %.

Determination of the Dimensional Stability

Paper samples having the dimensions 4 cm×4 cm were stored for one week over silica gel at a temperature of 20° C. in a desiccator. They were then weighed. In addition, the paper thickness (D₁) was determined. The samples were then stored over water for one week in a desiccator so that the paper was saturated with water vapor. The samples were then weighed and the thickness (D₂) of the samples was determined. The dimensional stability was determined as follows:

${{Dimensional}\mspace{14mu} {stability}} = {\frac{D_{2} - D_{1}}{D_{1}} \cdot {100\lbrack\%\rbrack}}$

In the formula, D₁ is the thickness of the dry paper and D₂ is the thickness of the moist paper.

Determination of the Moisture Absorption

Paper samples having the dimensions 4 cm×4 cm were stored for one week over silica gel at a temperature of 20° C. in a desiccator. Thereafter, they were weighed (W₁) and stored for one week over water in a desiccator so that the paper was saturated with water vapor. The samples were then weighed (W₂). The moisture absorption was determined as follows:

${{Moisture}\mspace{14mu} {absorption}}\mspace{14mu} = {\frac{W_{2} - W_{1}}{W_{1}} \cdot {100\lbrack\%\rbrack}}$

Determination of the Stiffness

The stiffness was determined by the beam method according to DIN 53 121. For this purpose, samples having a size of 100×25.4 mm were cut out of the papers to be tested, clamped, and measured under the following conditions: measuring length 1=100 mm, sample width b=25.4 mm, bending angle α=20°. It was ensured that a force F_(max) of at least 15 mN was reached at maximum deflection. Measurement was effected 5 times per sample.

Example 1

A paper stock which consisted of a mixture of 70% of bleached pine sulfate pulp and 30% of birch sulfate pulp and had a freeness of 35° SR (Schopper-Riegler) was drained in a Rapid Kö then sheet former. The sheets had a basis weight of 80 g/m². They were pressed in each case between filter papers to a water content of 50% and impregnated by first placing a sheet in a press whose base consisted of a paper machine wire having a mesh size of 100 μm, then placing in succession a papermaker's felt which was impregnated with a 50% strength aqueous solution of Kaurit® 210 (urea-formaldehyde resin) on the sheet and then covering with a sheet of polymethyl methacrylate. A pressure of 2.1 MPa was then exerted on the content of the press for period of 5 seconds. Water which originated from the paper and aqueous solution of the coating material from the papermaker's felt were forced from the bottom of the press. Thereafter, the pressure was canceled and the sheet impregnated in this manner was removed from the press. The sheet was dried for 4 hours at a temperature of 130° C. Under these conditions, the resin which was in the fibers and had been deposited thereon crosslinked. Stiffness, dimensional stability and water absorption of the sheet thus obtained were then tested. The results are shown in the table.

Examples 2-6

Example 1 was repeated with the only exception that in each case an aqueous solution and/or dispersion of the heat-curable binders shown in table 1 was used instead of the aqueous solution of Kaurit® 210. The results thus obtained are shown in table 2.

Comparative Example 1

Example 1 was repeated with the only exception that the papermaker's felt was now impregnated with distilled water instead of the aqueous solution of Kaurit® 210. The results are shown in table 2.

TABLE 1 Amount applied [g] Self-crosslinking material Example no. 1 8.0 Heat-curable urea-formaldehyde resin (Kaurit’® 210), 50% strength aqueous solution 2 7.5 Heat-curable melamine-formaldehyde resin (Kauramin 787), 60% strength aqueous solution 3 8.0 Mixture of polycarboxylic acid and polyfunctional amine (Acrodur ® 910L), 35% strength aqueous solution 4 8.2 Mixture of polycarboxylic acid and polyfunctional amine (Acrodur ® DS 3515), 35% strength aqueous dispersion 5 7.9 Heat-curable urea-formaldehyde resin (Fixapret ® ECO), 70% strength aqueous solution Comparative example no. 1 Impregnation with water

TABLE 2 Decrease in dimensional Moisture stability after Stiffness Water absorption moisture [mN] absorption [%] [%] absorption [%] Example no. 1 1700 100 15 3 2 2200 50 9 5 3 2000 270 20 6 4 3521 250 20 7 5 3000 110 15 4 Comparative example no. 1 254 475 22 8 

1. A method for reducing the absorption of water and water vapor and for increasing the dimensional stability of paper and paper products comprising: treating cellulose fibers with an aqueous solution and/or a dispersion of at least one reactive material which reacts with itself and/or the cellulose fibers via crosslinking, heating the treated cellulose fibers to a temperature at which drying and crosslinking takes place, compressing the cellulose fibers or a paper product obtained therefrom by drainage on a wire, contacting the compressed paper product with an aqueous solution and/or the dispersion of the reactive material, eliminating the compression with further action of the aqueous solution and/or the dispersion, and drying and crosslinking the paper product.
 2. The method according to claim 1, wherein the cellulose fibers comprise at least 50% by weight of virgin fibers or a paper product obtained therefrom by drainage on a wire, the cellulose fibers have a water content of at least 0.7 g of water per g of dry cellulose fibers, the cellulose fibers are compressed under a pressure of at least 2.1 MPa, and the paper product is dried and is heated to a temperature in the range of from 70 to 200° C. for crosslinking.
 3. The method according to claim 1, wherein the aqueous solution and/or the dispersion comprises, as reactive material, at least one heat-curable binder from the group consisting of urea-formaldehyde adducts, urea-glyoxal adducts, melamine-formaldehyde adducts, phenol-formaldehyde adducts, one- and two-component systems based on epoxy resins, polyurethanes or isocyanates, polyacrylates, polymethacrylates, styrene-(meth)acrylate copolymer dispersions and/or styrene-butadiene(meth)acrylic acid copolymer dispersions.
 4. The method according to claim 1, wherein the aqueous solution and/or the dispersion comprises, as reactive material, (i) at least one reactive substance which forms a polymer; (ii) optionally, at least one C₁₋₅-alcohol, at least one polyol or mixtures thereof; and (iii) at least one catalyst.
 5. The method according to claim 1, wherein the aqueous solution and/or the dispersion comprises, as reactive material, (i) at least one adduct selected from the group consisting of: at least one urea-formaldehyde adduct, one urea-glyoxal adduct and at least one melamine-formaldehyde adduct; (ii) optionally, at least one C₁₋₅-alcohol, at least one polyol or mixtures thereof; and (iii) at least one catalyst.
 6. The method according to claim 1, wherein the aqueous solution comprises, as reactive material, (i) 1,3-bis(hydroxymethyl)-4,5-dihydroxyimidazolidin-2-one.
 7. The method according to claim 1, wherein the aqueous solution and/or the dispersion comprises, as reactive material, (i) at least one compound selected from the group consisting of dimethylolurea, bis(methoxymethyl)urea, tetramethylolacetylenediurea, methylolmethyurea, 1,3-dimethyl-4,5-dihydroxyimidazonidin-2-one, 1,3-bis(hydroxymethyl)imidazolidin-2-one and mixtures thereof; (ii) optionally, at least one C₁₋₅-alcohol, at least one polyol or mixtures thereof; and (iii) at least one catalyst.
 8. The method according to claim 1, wherein the aqueous solution and/or the dispersion of the reactive material comprises at least one catalyst (iii) which is selected from the group consisting of metal halides, metal sulfates, metal nitrates, metal tetrafluoroborates and metal phosphates.
 9. The method according to claim 4, wherein the catalyst used is magnesium chloride.
 10. The method according to claim 1, wherein the reactive material used is a mixture of (a) a polymer which is obtained by free radical polymerization and which comprises, incorporated in the form of polymerized units, from 5 to 100% by weight of an ethylenically unsaturated carboxylic anhydride or of an ethylenically unsaturated dicarboxylic acid whose carboxyl groups can form an anhydride group, and (b) at least one alkanolamine which comprises at least two hydroxyl groups in the molecule and/or at least one polyhydric alcohol.
 11. The method according to claim 10, wherein aqueous mixtures of polycarboxylic acids and polyhydric alcohols and/or polyfunctional amines and/or alkanolamines are used as reactive material in amounts such that the number of acid functions is equivalent to the total number of alcoholic hydroxyl and amine functions.
 12. (canceled)
 13. A composition comprising: the paper products obtained by the method according to claim 1 and/or coated cellulose fibers produced by defibrating, and a thermoplastic.
 14. A composition comprising: the paper products obtained by the method according to claim 1 and/or coated cellulose fibers produced by defibrating, and a heat-curable plastic. 